A cryptocurrency (or less formally a coin) is a decentralized payment network with an independent currency-like asset that functions on the network and is essential to its function.

As opposed to government electronic money, cryptocurrencies use modern cryptography and decentralization to secure transactions and creation of monetary units. Cryptocurrency assets cannot be seized from their owners by a decree. Cryptocurrency transactions are global and cannot be easily censored.

In the first sense, we can say that "Cryptocurrency is a revolutionary invention." In the second sense, it is possible to say "I hold several cryptocurrencies on my Trezor."

Things should get a little bit more clear when reading about particular cryptocurrency, because of the following common practice of using uppercase and lowercase letter.

For example Bitcoin with uppercase B is used when referring to the Bitcoin as a network, system, software or protocol, while bitcoin with lowercase b is used referring to an asset, currency or its units. Thus it is quite clear what is meant in "Many people have realized that Bitcoin is a great way to send payments globally." as well as in "Most mainstream economists think that the bitcoin's fixed monetary supply is a bad idea."